[8] Like other horseshoe crabs, the carapace of T. gigas consists of a larger frontal one (the prosoma) and a smaller, spine-edged rear one (the opisthosoma).

The tail bears a crest dorsally and is concave ventrally,[2] giving it an essentially triangular cross section.

In common with other horseshoe crabs, T. gigas also has ventral eyes near the mouthparts, and photoreceptors in the caudal spine.

[7] In addition to their smaller size, males have a paler and rougher carapace, act as hosts to a greater number of epibionts,[8] have six (instead of three) long spines on either side of the rear carapace, and their two front pairs of walking legs, prosomal appendages two and three, have hooks (they are scissor-like in females).

[15] Tachypleus gigas inhabits seagrass meadows,[16] sandy and muddy shores[10] at depths to 40 m (130 ft);[2] it is the only horseshoe crab to have been observed swimming at the surface of the ocean.

[21] House crows have been observed to turn T. gigas over and eat the soft underside, while gulls only attack individuals that are already stranded upside-down.

[8] Among the larger organisms, the sea anemone Metridium, the bryozoan Membranipora, the barnacle Balanus amphitrite, and the bivalves Anomia and Crassostrea are the most frequent colonists of T.

[8] Rarer epibionts include green algae, flatworms, tunicates, isopods, amphipods, gastropods, mussels, pelecypods, annelids, and polychaetes.

[2] Tachypleus gigas is estimated to have diverged from the other Asian species of horseshoe crab 52.5 million years ago.